The present application is generally related to steering tools for horizontal directional drilling and, more particularly, to a system and method using supplemental magnetic information in a steering tool type arrangement.
A boring tool is well-known as a steerable drill head that can carry sensors, transmitters and associated electronics. The boring tool is usually controlled through a drill string that is extendable from a drill rig. The drill string is most often formed of drill pipe sections, which may be referred to hereinafter as drill rods, that are selectively attachable with one another for purposes of advancing and retracting the drill string. Steering is often accomplished using a beveled face on the drill head. Advancing the drill string while rotating should result in the drill head traveling straight forward, whereas advancing the drill string with the bevel oriented at some fixed angle will result in deflecting the drill head in some direction.
One approach that has been taken by the prior art for purposes of monitoring the progress of a boring tool in the field of horizontal directional drilling, resides in what is commonly referred to as a “steering tool”. This term has come to describe an overall system which essentially predicts the position of the boring tool, as it is advanced through the ground using a drill string, such that the boring tool can be steered toward a desired target or along a planned drill path within the ground. Steering tool systems are considered as being distinct from other types of locating systems used in horizontal directional drilling at least for the reason that the position of the boring tool is monitored in a step-wise fashion as it progresses through the ground. For this reason, positional error can accumulate with increasing progress through the ground up to unacceptable levels.
Generally, in a steering tool system, pitch and yaw angles of the drill-head are measured in coordination with extension of the drill string. From this, the drill-head position coordinates are obtained by numerical integration. Nominal or measured drill rod lengths can serve as a step size during integration. While this method appears to be sound and might enable an experienced driller to use the steering tool successfully, there are a number of concerns with respect to its operation, as will be discussed immediately hereinafter.
With respect to the aforementioned positional error, it is noted that this error can be attributed, at least in part, to pitch and yaw measurement errors that accumulate during integration. This can often result in large position errors after only a few hundred feet of drilling.
Another concern arises with respect to underground disturbances of the earth's magnetic field, which can cause significant yaw measurement bias errors, potentially leading to very inaccurate position estimates.
Still another concern arises to the extent that steering effectiveness of a typical HDD drill bit depends on many factors including drill bit design, mud flow rate and soil conditions. For example, attempting to steer in wet and sandy soil with the tool in the 12 o'clock roll position might become so ineffective that measured pitch does not provide correct vertical position changes. That is, the orientation of drill head, under such drilling conditions, does not necessarily reflect the direction of its travel.
One approach in dealing with the potential inaccuracy of the steering tool system is to confirm the position of the drill head independently. For example, the drill head can be fitted with a dipole transmitter. A walk over locator can then be used to receive the dipole field and independently locate the drill head. This approach is not always practical, for example, when drilling under a river, lake or freeway. In these situations, the operator might notice position errors too late during drilling and consequently might not have an opportunity to implement a drill-path correction.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.